This invention relates to a focusing device for a binocular instrument and more particularly to such a focusing device of the type in which right and left ocular lenses are moved forward and backward at the same time by a single focusing drive means. provided between the right and left ocular lenses. so as to achieve focusing. FIG. 6 shows a binocular instrument which includes a body 60 which can be flexed or bent about a central hinge 59. A pair of right and left arms 51a and 51b. which are commonly referred to as "wings", are connected respectively to ocular tubes 52a and 52b. A threaded rod 53 is connected to a joint between the pair of arms 51a and 51b. The arms 51a and 51b, the ocular tubes 52a and 52b, and the threaded rod 53 jointly constitute an ocular unit 50. Reference numerals 54a and 54b denote right and left ocular lenses, respectively. The pair of right and left arms 51a and 51b are angularly movable about the threaded rod 53 relative to each other.
A rotatable wheel 55 constituting a focusing drive means is mounted on the hinge 59 of the binocular body 60, and the rotatable wheel 55 is threaded on the threaded rod 53. By rotating the rotatable wheel 55. the ocular tubes 52a and 52b are moved forward and backward through the pair of arms 51a and 51b in the direction of the optical axis so as to achieve focusing.
As described above. the ocular unit 50 is supported at its central portion on the binocular body 60 through the threaded rod 53. One end of the two ocular tubes 52a and 52b are fitted respectively on annular engaging portions 56a and 56b formed on the ocular side of the binocular body 60, thus supporting the opposite ends of the ocular unit 50 on the binocular body 60.
That portion of each ocular tube 52a and 52b extending rearwardly from the backside of the arm 51a and 51b is fitted on the engaging portion 56a and 56b, respectively. When such a fitting portion (i.e.. area of engagement) of the ocular tubes 52a and 52b is made relatively long, the ocular portion projects considerably from the binocular body 60, which increases the size of the binocular instrument. Therefore, it is difficult to make the fitting portion long, and as a result the opposite ends of the ocular unit 50 connected to the binocular body 60 are slightly moved, due to a backlash, as indicated by arrows A (FIG. 6) on the central threaded rod 53 which in this case serves as a fulcrum. Therefore, when the user's face contacts the ocular portion during use of the binocular instrument, the ocular lenses 54a and 54b are moved, thus failing to obtain a good view of an object.
Further, the distance between the hinge 59 and the engaging portions 56a and 56b inevitably varies from one product to another. Therefore, if there is no degree of freedom in the distance B between the threaded rod 53 and each ocular tube 52a and 52b, it is possible that the fitting portion of the ocular tubes 52a and 52b may not be properly fitted on the engaging portions 56a and 56b when the threaded rod 53 is threaded into the rotatable wheel 55, thus failing to effectively mount the ocular unit 50 on the binocular body 60.
To deal with this problem, the right and left arms 51a and 51b can be loosely connected to the threaded rod 53 so that the former can slightly move with respect to the latter. With this arrangement, the distance B between the threaded rod 53 and the ocular tubes 52a and 52b can be varied in an amount corresponding to such play.
However, when the arm 51a or 51b to which the ocular tube 52a or 52b is connected has such a play, the ocular lens 54a and 54b is moved or displaced when it comes into contact with the user's face during use of the binocular instrument, thus failing to obtain a good view of an object.
Further, in the above focusing device for a binocular instrument. the ocular unit 50 is provided exterior to the binocular body 60 and hence is exposed to the exterior Therefore foreign matters such as dirt, dust and sand tend to deposit on the operating portion which achieves focusing, causing improper operation. Also. the operating portion can be easily damaged or broken when subjected to an impact such as when the binocular instrument is dropped, thus causing a malfunction.